As negotiations resume Wednesday in Baghdad between Iran and the five permanent members of the United Nations Security Council plus Germany (the "P5+1"), there are growing hopes for a diplomatic breakthrough over Tehran's nuclear ambitions. This sense of optimism has been buoyed by the hopeful statements of the director general of the International Atomic Energy Agency (IAEA) after his visit to Tehran this week.

We want to be hopeful, too. A negotiated settlement that verifiably ends Iran's illicit nuclear activities and prevents Iran from possessing the capability to assemble a nuclear weapon quickly is desirable and possible. But we must not allow these talks to become a movie we've seen before, in which success is defined less by the outcome of negotiations than by their mere perpetuation.

The Iranian regime's long record of deceit and defiance should make us extremely cautious about its willingness to engage in good-faith diplomacy. And its nuclear pursuit cannot be divorced from its other destabilizing actions—support for violent extremist groups such as Hezbollah and the Taliban, threats against Arab governments and Israel, attempts to assassinate foreign diplomats, and lethal assistance to the Assad regime in Syria.

In fact, Iran's new-found interest in negotiating is almost certainly a result of the strong pressure that the regime now faces from economic sanctions. Most important of all have been U.S. and European Union efforts to obstruct Iran's ability to derive revenue from international oil sales—a campaign whose full brunt won't be felt until later this summer.

Based on its past behavior, we should expect Iran's government to use the talks to buy time, undermine international unity, and relieve the mounting economic pressure it faces. The U.S., in turn, must work with our partners to make clear that there will be no diminution of pressure until the totality of Iran's illicit nuclear activities has been addressed.

That will require much more than shuttering the underground enrichment facility at Fordow, removing from Iranian territory all uranium enriched to 20%, and suspending further enrichment at that level—the three steps that reports suggest the P5+1 negotiators will emphasize in Baghdad.

Remember that Iran had no uranium enriched to 20% until two years ago, nor was the Fordow site operational before then. Focusing only on these recent manifestations of Iran's nuclear program, without also addressing older and broader enrichment and proliferation-sensitive activities, would effectively reward the Iranians for their escalation and allow them to move back the goal posts.

Rather, the U.S. must make clear that international pressure will continue to build on Iran until it takes the concrete steps that will address the entirety of the threat, with a swift timetable for implementation. These must include:

• Full Iranian cooperation with the IAEA—not just promises to cooperate, but tangible action to resolve all outstanding questions about Iran's illicit nuclear activities.
• A new agreement to intrusive inspections based on the Additional Protocol under the Nuclear Non-Proliferation Treaty to ensure the Iranians aren't lying or cheating about the full scope of their program, as they have in the past.
• Full Iranian compliance with all resolutions of the U.N. Security Council, including its repeated demand for full, verifiable and sustained suspension of all enrichment-related, reprocessing and heavy-water activities.

Given the Iranian regime's long-standing pattern of deceptive and illicit conduct, we believe it cannot be trusted to maintain enrichment or reprocessing activities on its territory for the foreseeable future—at least until the international community has been fully convinced that Iran has decided to abandon any nuclear-weapons ambitions. We are very far from that point.

Similarly, and just as importantly, Iran must not be permitted to possess sufficient fissile material for a nuclear weapon, or centrifuges in sufficient quantity or sophistication that would allow it to "break out" and build a nuclear weapon swiftly and covertly.

A diplomatic solution with Iran is possible if the Iranian regime genuinely wants one. But to achieve this outcome, we must not allow the Iranians to draw us into an extended negotiation with a continuing series of confidence-building measures that never ultimately force Tehran to verifiably abandon its pursuit of a nuclear-weapons capability. We've been sold that horse many times before, most notably in the failed efforts over two decades to end the North Korean nuclear program.

Our best hope for avoiding conflict is to leave no doubt that the window for diplomacy is closing. In the absence of a negotiated solution that addresses the totality of Iran's nuclear program, and soon, we must take the steps that President Obama laid out in February, when he said: "America is determined to prevent Iran from getting a nuclear weapon, and I will take no options off the table to achieve that goal." The U.S. must be prepared, if necessary, to use military force to stop Iran from getting a nuclear-weapons capability. 

The meetings in Baghdad could be one of our best and last chances to peacefully resolve the Iranian regime's pursuit of a nuclear-weapons capability. But this opportunity will be lost if we allow Iran's negotiators to fool us into easing the pressure before the Tehran regime has truly abandoned its military nuclear ambitions. 

This OpEd appeared in the Wall Street Journal on May 23, 2012.  Graham and McCain are Republican senators from South Carolina and Arizona, respectively. Mr. Lieberman is an Independent Democratic senator from Connecticut.